Networks Horizon

Q1. The use of a loopback interface to define neighbors is common with iBGP, but is not common with eBGP. Normally, you use the loopback interface to make sure that the IP address of the neighbor stays up and is independent of hardware that functions properly. In the case of eBGP, peer routers frequently have direct connection, and loopback does not apply.
is this true that we don't use loopback for eBGP.

Ans: It is not so that we never use loopback address on eBGP neighborship . Sometimes, there might be scenario where you have two or more redundant path between two eBGP peers.

link:

                     s0...................................s0 

loopback-----R1                                        R2-------loopback

                     s1...................................s1

To understand this first we need to understand why we need loopback addresses for IBGP connectivity.

Similarly in eBGP there may be the case if one of the two available paths is un-available,  BGP neighborship will remain intact through loopback addresses because IP connectivity will remain there through alternate path.

By the rule, eBGP neighborship should be directly connected but there may be another scenario where we should always use loopback address to create eBGP neighborship to utilize all the parralel available links( incase we have two or more direct links).

  

neighbor ebgp-multihop, neighbor ttl-security hops commands are used where neighbors are not directly connected (neighbors are multiple hops away)

neighbor disabled-connected command should be used where eBGP neighborship is formed via loopback address.

  

like iBGP neighborship here also update-source <loopback int> should be changed alogwith. Also needs to remember that loopback interfaces are not physical interfaces and always remains up unless whole router is down.

Alternatively dynamic static routing can also be used instead of using BGP for the purpose . Irrespective of BGP power, we should prefer static routing over eBGP which is easiler to configure and provides less CPU overhead.

  

ISP environment

While the external BGP (EBGP) sessions are usually established between directly-connected routers, IBGP sessions are expected to be configured across the network.

Best iBGP Practices: Therefore, the current best practice is to configure IBGP sessions between the loopback interfaces of the BGP neighbors, ensuring that the TCP session between them (and the BGP adjacency using the TCP session) will not be disrupted after a physical link failure as long as there is an alternate path toward the adjacent router.

Best eBGP Practices: Establish EBGP session on physical link and not via loopback interfaces unless nee there are more links between two EBGP